The present invention relates to heat treating coils of metal sheet in furnaces
Coiled sheets of metal are customarily annealed in batches in batch-type furnaces. Such annealing involves placing several large coils on a suitable carrier and disposing the carrier with the coils in the furnace. The coils are heated to a desired annealing temperature, usually between 650.degree. and 900.degree. F., in the case of aluminum, and allowed to soak for an interval of several hours to effect the desired anneal in the mass of each coil and thus in the product readied for shipment to customers. Annealing is required because of the work hardening that occurs when the sheet of the coil is rolled in the process of reducing its thickness, i.e., the sheet is worked hardened to a degree requiring it to be softened for further working or shaping.
Over the years, the size of aluminum coils supplied to the container industry, for example, have become increasingly large. The width of the sheet that makes up the coil can be as large as sixty three inches, with an outside diameter of ninety inches. The weight of such a coil is about 50,000 pounds. The container industry requires that the coiled material be partially annealed so that the container forming process can be effected with maximum ease, which occurs on a mass production basis. This requires close control of the metallurgical properties of the material of the coil, and these properties are obtained by heat treating coils for a period of time and in a controlled manner to effect such properties.
Measuring the surface temperature of heated workpieces after they have been heated for a time period sufficient to allow uniformity of temperature throughout the masses of the workpieces, can be accomplished by the use of thermocouples inserted into the workpiece surfaces. Such a procedure is shown and described in U.S. Pat. No. 3,447,790 to Ross et al. This requires the drilling of holes in the workpiece to receive the thermocouples.
In the case of the large coils discussed above, the thermocouples are inserted into the edges of the wraps of sheet material that form each coil so that the temperature measured by the thermocouples will be closer to the center of the mass of the coil, i.e., the heat of the furnace atmosphere begins at the outer sides or edges of the coil and migrates into the wraps of the coil until the coil mass is heated.
Wires are then strung between the thermocouples and instruments for reading the electrical output of the thermocouples, the electrical output being indicative of workpiece temperature. It can be appreciated that such a procedure is slow and labor intensive and requires, in addition, the damaging of the edges of each coil. Further, the failure rate of thermocouples can be high. It can further be appreciated that in production facilities where time is an important parameter in reducing unit costs and increasing profits, the attachment of thermocouples to workpieces is not a desirable way to determine and monitor workpiece temperature.
Another method of measuring surface temperature is the use of infrared sensing devices, which devices do not contact (and damage) the workpiece but rather receive heat that is radiated from a heated surface. These devices, however, have had major weaknesses, particularly in measuring the temperature of aluminum workpieces, because of the low and changing emissivity constant of aluminum surfaces. Such emissivity causes errors in the readings of infrared detectors. Additional error is associated with radiation that is redirected from furnace walls to the detectors.
The emissivity of an aluminum surface is also a problem in infrared measuring. Emissivity is related to incident radiation, i.e., radiation that is reflected by a body, by the equation E+R=1. When emissivity E is unity, reflectivity R is zero, the emitted energy being directly related to true surface temperature. However, non-perfect radiators, such as heated aluminum surfaces, are partial reflectors of radiation such that their emissivity is always less than unity. Hence, the total energy leaving an area of such a body will, in general, not provide a reading of true surface temperature unless its emissivity is determined beforehand and an appropriate correction applied.
What is generally known as a "black body" and a perfect radiator is characterized by the fact that the energy which the black body and radiator emits depends only upon its absolute temperature. (The perfect radiator is called "black" because of the color [wavelength] that the human eye perceives in the visible spectrum; as explained hereinafter, the perfect radiator in the infrared spectrum is not necessarily "black".)
A non-black or gray body radiator emits then only a fraction of the energy emitted by a perfect radiator, the fraction being dependent on the "emissivity" of the body surface. Thus, in order to relate energy emitted by a non-perfect radiator to true temperature, the prior art has either had to provide means to compensate for the emissivity of the non-perfect or gray radiator or use a coating of black body material on the workpiece. U.S. Pat. No. 4,465,382 to Iuchi et al and U.S. Pat. No. 4,659,234 to Brouwer et al are examples of the compensating method, while U.S. Pat. No. 4,408,903 to Baldasarri and a paper entitled "A Beginner's Guide to Infra-Red Thermometers" by J. M. Rucklidge (Land Instruments, Inc. 1979) disclose the second method.